For the past 40 years, air navigation has primarily consisted of various forms of radio direction finding devices. With these devices, navigation information is transmitted from a fixed ground station and received by airborne aircraft equipped with the appropriate receivers. Each ground transmitter has a unique radio frequency associated therewith. To navigate an aircraft, the pilot dials or tunes a receiver to the frequency associated with the ground based transmitter, and then flies the aircraft towards the transmitter. Once the pilot reaches the transmitter, the pilot tunes in the frequency of the next transmitter along the pilot's chosen route. Identifiers for the ground transmitters are typically displayed to the pilot as TO navaids and FROM navaids. An identifier for the ground transmitter towards which the aircraft is heading is marked as the TO waypoint or navaid. Similarly, the last ground transmitter from which the aircraft is heading is marked as the FROM waypoint or navaid. Thus, prior art radio navigation systems require the pilot to tune a receiver to a particular frequency and to then fly the aircraft from transmitter to transmitter. Common radio navigation transmission systems used by the pilot community include VORs, DMEs, TACANs, and NDBs.
VOR is an acronym for very high frequency omnidirectional range. It is the Federal Aviation Administration's (FAA's) very high frequency (VHF) based point-to-point navigation system. VOR consists of a ground station transmitter and an airborne VOR receiver. The ground transmitter transmits phase encoded signals outward from the transmitter in all directions. The airborne receiver receives the transmitted VOR signal and decodes the phase information to determine the aircraft's bearing with respect to the ground transmitter. The aircraft's bearing is referred as being on a particular "radial" from the VOR transmitter. Radial information is commonly displayed on a course deviation indicator (CDI) gauge or on a radio magnetic indicator (RMI) gauge. VOR is a line of sight transmission system. As a result, VOR range is typically limited to 130 nautical miles at best due to the curvature of the earth. However, other obstructions can further limit the range of conventional VOR systems. Additionally, intrinsic VOR system errors contribute substantial error to VOR readings. At a maximum range from the VOR transmitter, errors of as much as 20 nautical miles are possible.
DME is an acronym for distance measuring equipment. DME is an active system requiring receivers and transmitters at both the ground station and in the airborne aircraft. A DME system is initiated by the airborne unit sending ultra high frequency (UHF) pulses to the ground station and the ground station sending responding UHF pulses back to the airborne unit's receiver. The airborne unit measures the time interval between the initial transmission and receipt of the responding message. The measured time is used to calculate the distance of the aircraft from the DME station. Typically, DME stations are co-located with VOR stations in a VOR/DME station. As with VOR stations DME systems also suffer from significant error. Furthermore, due to the interactive nature of DME systems, DME stations can become overloaded in congested airspace environments.
TACAN is an acronym for tactical air navigation. TACAN is the military counterpart to combination VOR/DME stations. TACAN operation is very similar to VOR/DME operation, where the pilot receives both direction and range indications on the aircraft instrument gauges. TACAN, like other radio navigation systems, has error associated therewith. In fact, TACAN accuracy is only slightly better than VOR/DME.
NDB is the acronym for non-directional radio beacons. Although NDBs are typically not used for general air navigation in the continental United States, NDBs are still used in many less developed regions of the world. Thus, NDBs remain an important part of instrument approaches for many pilots. Pilots typically use NDBs as compass locators to aid in finding the initial approach point of an instrument landing system. NDBs are also used for nonprecision approaches at low-traffic density airports without conventional VOR approaches. In a NDB system, the direction, or bearing, of the aircraft with respect to the transmitting ground station is generally displayed on a compass card gauge by means of a pointer. An NDB systems is not as accurate as a VOR system. Additionally, NDB radio signals are subject to many propagational and atmospheric degradations which further reduce the accuracy of the NDB system. Hence existing radio navigation systems have considerable errors and inaccuracies associated therewith.
In addition to being familiar with certain radio navigation systems, pilots have been extensively trained on certain radio navigation instrumentation devices. As a result of their initial training and ongoing use of such radio navigation devices, many pilots resist using new navigation systems and/or new navigation instrumentation devices. Thus, even though conventional radio navigation systems may have errors and inaccuracies associated therewith many pilots are reluctant to give up familiar instrumentation devices.
As yet another drawback, in a radio navigation system, a pilot navigates the aircraft along a route which extends from one radio transmitter to another radio transmitter and so on, until the aircraft reaches the desired location. As a result pilots are often forced to travel along a circuitous route to reach a desired destination. Prior Art FIG. 1 is an example of an airway navigation en route chart. On the chart, airways are represented as lines between stations 100. For example, to fly from Helena 102 to Jackson 104 using the airway system, a pilot would fly from Helena 102 to Whitehall 104 via airway V21 106. The pilot would then fly the aircraft from Whitehall 104 to Dillon 108 via V21 110. Next the pilot would fly from Dillon 108 to Dubois 112 via V21 114. Finally, the pilot would fly from Dubois 112 to Jackson 116 via V298 118.
In an attempt to overcome shortcomings associated with radio navigation systems, a navigation system employing the Global Positioning System (GPS) had been introduced. The use of a GPS based aircraft navigation system is intended to eliminate the circuitous navaid to navaid scheme used in radio navigation systems, and improve navigation accuracy. GPS based navigation systems allow a pilot to fly from a point of origin directly to a destination. Thus, GPS systems eliminate circuitous navaid to navaid routing schemes of radio navigation systems.
However, the conventional radio navigation airway system has been in operation since the late 1940s. As a result, there are literally hundreds of thousands of pilots who were trained in radio navigation using standardized radio navigation instrumentation devices. The user interface in radio navigation equipment, due both to the length of time the equipment has been in use and the widespread standardization of the display configuration, is basically the same now as it was 30 years ago. Hence, there is great reluctance in the pilot community to use new GPS based navigation aids.
Additionally, aircraft built by different manufacturers have contained standardized radio navigation equipment. Therefore, pilots could easily switch from an aircraft built by one manufacturer into an aircraft built by another manufacturer. With GPS navigation receivers, different units built by different manufacturers are to varying degrees, unique. That is, each manufacturer has its own special control/display arrangements. Thus, in order to effectively use these GPS products, the pilot must have knowledge of nested menus, numerous buttons and knobs, and various new functions. Therefore, the pilot must learn a completely new operating system in order effectively use present GPS based navigation devices.
Also, in order to safely use the GPS navaids, extensive training is often required to master the myriad of informational display techniques and control inputs/outputs. Almost none of this training investment is transferable to other GPS navaids, since each manufacturer follows his own protocols and configurations when designing their systems. Training necessary to safely and. properly operate these new GPS navaids is highly specialized and is very expensive. Furthermore, very few flight instructors in the general aviation community have experience with the newer models of GPS navaids. In addition, most general aviation pilots have previously learned to think of navigation in terms of radial and distance when navigating with radio navigation based equipment. To use GPS, pilots must learn to think in terms of latitude and longitude map coordinates. Thus, current GPS navigation products from avionics manufacturers are often difficult to use in the actual inflight environment. As a result, a very significant safety issue arises when attempting to transition pilots from conventional radio navigation to GPS based navigation.
Thus, a need has arisen for a navigation device which eliminates the errors associated with radio navigation systems and provides GPS level accuracy. A further need exists for uniform and standardized navigation instrumentation for use with a navigation system providing GPS level accuracy. Yet another need exists for navigation instrumentation which operates with GPS level accuracy and which the pilot community will not be reluctant to adopt.